Add other (non-stock) items to Accounts & Sync list

I've tried searching around Google and this forum and couldn't find an answer to this question, so figured I'd post.

Basically I do all my syncing manually, whereby I have a number of items in my Accounts & Sync list (a Facebook account, two Gmail accounts, Twitter, etc) get synced when I hit a shortcut on my desktop. These all work fine.

My problem is that I have other things that I want to sync as well - namely the Weather Channel app, the other Facebook app (the one that isn't Friendstream), Tripit, etc., and it seems the only way to sync these manually is to open the program and hit sync/refresh. I don't want to use a scheduled sync.

I want to be able to add them to the main list in 'Settings > Accounts & sync' so that I can do all my syncs in the one go, the native/official ones as well as these other ones, but for the life of me I can't figure out how to do this.